deserialize mappings can now handle nested objects and can specify a mapping for each nested model
updated import_data not to overwrite the existing data
Merge branch 'nested_deserialize_mapping' into updated_deserialize_mapping
Document the creation of custom types
Change standard coerce_key function to unicode
Convert bare expressions to assertions

There were several instances of tests like `foo == 23`, but nothing really
happens if that's untrue. Python discards the results of expressions that
aren't assigned or otherwise used. This patch converts those to assertions
so that the tests will fail loudly when they don't pass, instead of silently
continuing on to the next statement.
Minor pylint cleanups

Unused imports, assignment to variables that aren't used, replacing
`foo != None` with `foo is not None`, extraneous whitespace.

Nothing show-stopping, but we might as well make it look pretty. :-)
Giving context to serialization

This adds a new 'context' keyword argument to all to_native,
to_primitive, and related function so that users can pass in information
that those methods need to correctly deserialize data.

For instance, suppose a hypothetical MultiLanguageString type that holds
messages in many different locales. The context argument allows users to
pass in a requested locale so that the MultiLanguageString can return
the correct string representation. For example:

```python
    >>> class TestModel(Model):
    ...     mls = MultiLingualStringType()
    ...

    >>> t = TestModel({'mls': {'en_US': 'Hello, world!'}})

    >>> t.to_native()
    {'mls': {u'en_US': u'Hello, world!'}}

    >>> t.to_native(context={'locale': 'en_US'})
    {'mls': u'Hello, world!'}
```
Fixed typo in variable name
Bumped pytest in dev-requirements to match requirements-testing
Remove dynamically generated .coverage file from repo
Added context documentation
Added MultilingualStringType and tests
It seems I indent weirdly when sleepy
Accept lists of locales, too
Added more MultilingualStringType docs
"langauge" => "language"
.to_primitive is the workhorse, not .to_native
Ensure dict- or None-ness in to_native
Updated exporting docs with real context example

Now that we have an actual MultilingualStringType, we can use it for
our "Using Contexts" example.